rename the service to diff.sr.ht

The instance-facing name changes and nothing else does: the config.ini section,
the origin, the apk package, the nginx block and the systemd unit are all
diff.sr.ht now, and the brand's red service label reads "diff" because chrome
derives it from the section name.

What deliberately stays is everything whose rename would cost more than it is
worth: the Go module path, the repository name, the comparesrht binary, and the
/~owner/repo/compare/base...head routes people have in their history. README
says so at the top so the split is not read as an oversight.

compare.sr.ht: project foundation — core package, build scaffolding, core-go fork pin

Bootstrap the go.bigb.es/sourcehut-compare service:

- go.mod (go 1.26.4) pinning core-go to the private fork
  git.srht.bigb.es/~bigbes/core-go @ c2c2f3848fa9 via a replace directive;
  go.sum populated by a throwaway smoke build importing core-go
  config/crypto/client/server + chi + logrus (gates API drift from the
  verification baseline fdb3662 to upstream 71b2787).
- core/: pure domain package — sentinel errors, owner/repo/ref validation
  (check-ref-format-style, hostile-input hardened), and the compare-spec
  grammar (ParseCompareSpec, three-dot/two-dot, percent-unescape). Table
  tests at 97% coverage.
- Makefile (all/build/test/css/bundle/run-dev/install), config.example.ini
  documenting the shared keys read in place, contrib/ nginx block and
  systemd unit, README skeleton, .gitignore.

core-go dep deps and go mod tidy are deferred to later phases per plan.